"The flow rate for *all* the appliances in the house is limited by the capacity of the rising main. Flushing a toilet may thus lead to the hot water temperature fluctuating, or even going cold. Do not install a combination boiler unless your rising main is sufficiently good - flow rate may not be adequate for washing machines/dishwashers that require a hot feed."
At this place...
http://www.diyfaq.org.uk/plumbing/combi.html
Peccavi
April 2009
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